The present invention relates to an industrial method of producing high HLB sugar fatty acid esters.
At present, sugar fatty acid esters (hereinafter called also &lt;&lt;SE&gt;&gt; for short) useful for surfactants are industrially obtained by the reaction of sugar and higher fatty acid methyl esters of C.sub.8 to C.sub.22 in solvents (for example dimethyl formamide, dimethyl sulfoxide and the like) in the presence of suitable catalysts (the solvent method: Japanese Patent Publication No. Sho 35-13102) or by melting sugar together with fatty acid soaps without using solvents but using water and then acting the molten mixture upon higher fatty acid methyl esters in the presence of catalysts (the water medium method: Japanese Patent Publication No. Sho 51-14485).
However, according to these two kinds of synthetic method, impurities, such as unreacted sugars, unreacted fatty acid methyl esters, remaining catalysts, soaps, free fatty acids and volatile matter (the remaining reaction solvent) used in the above mentioned "the solvent method" of SE synthesis are contained in the reaction mixture in addition to the aimed SE and the impurities, of which contents exceed the regulation values, must be removed before the products are obtained. In particular, of the above described impurities, the removal of the remaining reaction solvents (volatile matter) in the former solvent method is remarkably important in view of the recent increasingly severe regulation.* FNT * According to the standard of the FDA in U.S.A., the permissible concentration of remaining dimethyl sulfoxide in SE is 2 ppm or less [Fed. Regist., 51(214), 40160-1].
So, in order to "remove the impurities such as the remaining and others from the crude SE", a large amount of organic solvents (for example butanol, toluene, methylethyl ketone, methyl acetate and the like; refer to Japanese Patent Publication No. Sho 42-11588, Sho 48-1-448 and the like) has been used but the use of such a large amount of solvents has led to the following remarkable disadvantages in the industrial production of SE.
1) The possibility of the explosion and fire;
2) The provision of explosion proof electric apparatus against the above 1);
3) The tight closure of the manufacturing apparatus against the above 1);
4) The provision of the fire resisting construction all over the building against the above 1);
5) The rise of fixed cost due to the above 2), 3), 4).
6) The rise of cost price due to the loss of the solvents;
7) The negative effect due to the remaining solvents in product SE;
8) The bad influences upon the health of workers and thus the rise of fixed cost due to said bad influences.
In view of such the circumstances, it has been strongly desired in the industrial field in question to develop an art of making the use of organic solvents unnecessary in the purification of SE.
It is true that the purification methods without utilizing organic solvents have been formerly investigated experimentally and the following representative methods have been known.
1) The method of sedimenting SE with an acidic aqueous solution [British Patent 809,815 (1959)];
2) The method of sedimenting SE with an aqueous solution of general neutral salts (Japanese Patent Publication No. Sho 42-8850); and the like.
But if for example an aqueous solution of hydrochloric acid is added to a reaction mixture, as in the method 1), although SE is immediately sedimented, unreacted sugar is easily decomposed and transformed into glucose and fructose. Even though the main operation is conducted at low temperatures (0.degree. to 5.degree. C.), the decomposition can not be avoided. Thus, the recovery and reuse of the unreacted sugar become remarkably difficult.
In addition, even though the aqueous solution of neutral salts, such as sodium chloride and sodium sulfate, is added to the reaction mixture, SE is immediately sedimented as in the method 2). In this case, the unreacted sugar is not decomposed but monoesters in SE, which are useful ingredients are more soluble in water than other diesters, triesters and so on in SE are dissolved in a water phase side, so that not only the loss is increased but also the obtainment of sugar fatty acid esters (hereinafter referred to also&lt;&lt;high HLB-SE&gt;&gt;) having the high HLB value *, which are in particular recently great demand and the object of the present invention, is hindered. FNT * The hydrophile-lipophile balance having a value within a range of about 1 to 20. The larger this value is, the stronger the hydrophilicity is.
Furthermore, according to the more recent Japanese Patent Application Laid-Open No. Sho 51-29417, the property of separating the mixture solution of water and "purification solvents" (called in this manner is particular for discriminating from the reaction solvents) into a light liquid layer (a solvent phase side=upper layer) and a heavy liquid layer (a water phase side=lower layer) is used for the purification. That is to say, since in general a large quantity of water is contained in the heavy liquid layer (a water phase side=lower layer), the hydrophilic unreacted sugar, salts resulting from the catalysts and the like are dissolved in the heavy liquid layer (a water phase side=lower layer) while since a large quantity of purification solvents is contained in the light liquid layer (a solvent phase side=upper layer), the substances, such as SE, fatty acids and unreacted fatty acid methyl esters, having low polarity are dissolved in this light liquid layer (a solvent phase side=upper layer). Accordingly, unreacted sugar, salts and the like in the heavy liquid layer (a water phase side=lower layer) can be separated from SE, fatty acid and unreacted fatty acid methyl esters in the light liquid layer (a solvent phase side=upper layer) by the phase separation.
In this case, the reaction solvents, such as dimethyl sulfoxide which is used in SE synthesis are dissolved in the lower heavy liquid layer (the water phase side=lower layer) but they are dissolved in also the upper light liquid layer (a solvent phase side=upper layer) inconveniently. In some cases, it is possible to remove the reaction solvents by this method from SE. With the aid of difference between the solubilities of the reaction solvents in the lower layer and upper layer, many times of this phase separation procedure) makes it possible to remove most of reaction solvents for example to 2 ppm of Dimethyl sulfoxide. However this procedure necessitates a remarkable large quantity of purification solvent for removing the reaction solvent perfectly and is expensive in industrial manufacturing of SE. Thus, the known SE purification method has some disadvantages as formerly mentioned, i.e. use of purification solvent, possibility of fire, cost of the exprosion electric apparatus, rise of fixed cost, negative effect due to the remaining solvents in product SE and bad influence upon the health of workers.
In order to improve the known purification method of crude SE industrially in the different manner from the above described, inventors have been researched for several years the use of mere water as purification solvent instead of organic solvent, and it is achieved commercially as opened here. New invented method is firstly characterized by the mere water use instead of organic solvent as purification solvent i.e. nonorganic solvent purification of crude SE. Such a method has been never developed commercially. It is required to purify crude SE capable of perfectly removing the reaction solvents without bringing about the loss of the sugars and the product SE with mere water.
Furthermore, a problem to be solved in order to make the purification of SE with mere water industrially possible occurs in the drying of SE resulting from the mere use of water as the purification solvents.
That is to say, in usual the mere water-contained SE to be dried here is an aqueous solution when the water-content is 80% or more and a slurry when the water-content is less than 80%. Such the mere water-contained SE exhibits a remarkably peculiar viscosity behavior that in general the viscosity rapidly rises from about 40.degree. C. with a maximum value at about 50.degree. C. and is rapidly reduced when the temperature exceeds 50.degree. C. (refer to pages 103, 107, 108, &lt;&lt;The Story of Sugar Esters (1984)&gt;&gt; published by the applicant company). In addition, it is substantially practically impossible due to the remarkable foaming to evaporate water by heating in a vacuum. And, if the heating temperature is high and the time of contact with the heating member is long, not only the decomposition of SE and the strong coloring and caramellization occur and also the acid value rises by the free fatty acids increased by the decomposition of SE (refer to Japanese Patent Publication No. Sho 37-9966).
In particular, in the end stage of the evaporation of water, the SE itself has a tendency of foaming with remaining water due to its characteristics that the softening point or the melting point is low (for example the softening point of sugar monostearate is about 52.degree. C. and the melting point of sugar distearate is about 110.degree. C.) and this leads to the more difficult dehydration and drying of SE. In addition, also the remarkably higher latent vaporization heat (c.a. 500 Kcal/kg H.sub.2 O or more). The higher vaporization temperature of water and the like than those of the solvents lead to the more difficult dryability. Accordingly, also in the case where the so-called "flash type" drier, into which the SE slurry heated by steam is continuously supplied, flashed in the vacuum chamber, dehydrated and dried, the sufficient dehydration and drying is haunted by various kinds of difficulty as above mentioned and even though these difficulties can be overcome, the dehydrated and dried SE is kept under the molten condition by heating, and that a large number of steps, such as a step of taking the molten SE out of the drier, a step of cooling the taken-out molten SE until temperatures of the melting point or less to solidify by blowing cool air and the like and the final pulverization in the pulverizer, are required and there are not only the degradation of SE due to many procedure steps, but also the possibility of the dust explosion in the final pulverization step.
Accordingly, also the solution of the above described various kinds of problem resulting from the drying is an important step for realizing the purification according to the present mere water medium purification method of crude SE. Thus new invited method is 2ndly characteristic of improved SE drying manner different from known SE manufacturing method. Such a SE drying method has never been developed commercially.